PROJECT SUMMARY The overall goal of the PLAY (Play & Learning Across a Year) project is to catalyze discovery about behavioral development in infancy. PLAY will focus on the critical period from 12 to 24 months of age when infants show remarkable advances in language, object interaction, locomotion, and emotion regulation. PLAY will leverage the joint expertise of 63 ?launch group? researchers, and capitalize on the Databrary video-sharing library and Datavyu video-coding tool to exploit the power of video to reveal the richness and complexity of behavior. Together, PLAY researchers will collect, transcribe, code, share, and use a video corpus of infant and mother naturalistic activity in the home to test behavioral, developmental, and environmental cascades. The project will demonstrate the value and feasibility of a cross-domain synergistic approach, and advance new ways to use video as documentation to facilitate discovery and ensure transparency and reproducibility. Aim 1 is to create the first, cross-domain, large-scale, transcribed, coded, and curated video corpus of human behavior?collected with a common protocol and coded with common criteria jointly developed by the launch group. The corpus will consist of videos of 900 infant-mother dyads (300 12-, 18-, and 24-month-olds) from 30 diverse sites across the United States. Videos will be transcribed and coded for infant and mother communicative acts, gestures, object interactions, locomotion, and emotion. The corpus will be augmented with video home tours and questionnaire data on infant language, temperament, locomotion/fall injuries, gender identity and socialization; home environment and media use; and family health and demographics. Aim 2 is to leverage the potential of time-locked video codes to test critical questions about behavioral, developmental, and environmental cascades?from one domain to another, between infants and mothers, and from the macro environment (e.g., SES, geographic region, home language) and proximal home environment (e.g., objects for play, home chaos and clutter) to infant and mother behaviors. Aim 3 is to advance new ways to use video as documentation to ensure scientific transparency and reproducibility. The entire protocol and code definitions are documented in a wiki with exemplar video clips to illustrate text-based descriptions. The entire corpus and all tools will be openly shared with the developmental and behavioral science communities on Databrary and in other language and behavior repositories (CHILDES, HomeBank, WordBank, OSF). PLAY will create a cross-domain, shared video corpus of unprecedented scope and richness. It will provide launch group members and the larger research community with the data, tools, and know-how to use time-locked video codes to investigate the unfolding of natural behavior in real time. The novel, synergistic approach to crowdsourcing the research will reduce overall costs while increasing scientific payoffs. The use of shared video as both data and documentation will accelerate the pace of discovery in developmental science.